Anhydrous ammonia is not only the nitrogen source for most synthetic fertilizers but also is commonly applied directly to the ground in its natural state for grain crops (corn, milo, wheat, etc.).
Ammonia is typically stored in a pressure vessel in its liquified gas phase, where it is a liquid due to its own vapor pressure. Heat flow into the system will be absorbed in the system by additional formation of vapor, increasing system pressure. Heat flow out of the system will produce condensation of vapor, reducing system pressure.
The application and distribution of ammonia for fertilizer is usually accomplished by a tractor equipped with a tool bar or cultivator. The ammonia is metered and placed into a plurality of spaced rows, 4 inches to 8 inches beneath the soil's surface, and the soil is sealed. Typically, a tractor tows an ammonia nurse tank wagon behind the tool bar, and suitable flexible connections are provided to transfer ammonia from the tank through a meter on the tool bar into a manifold, where flow is divided and fed into the soil through hoses and applicator knives.
The energy required to move ammonia through the system is supplied by its vapor pressure in the tank, however, ammonia moving from the tank to the meter experiences a pressure drop equal to the pressure head required to accelerate ammonia to its velocity through the system. This pressure drop requires a temperature reduction in the system which is provided by refrigeration or vaporization within the system. The colder the day, the lower the tank pressure, and the greater the distance from the tank to the meter--the greater the percentage of vapor in the system prior to metering. Vapor in the system prior to metering degrades the accuracy of the metering function. Operations using larger, more powerful tractors available today, capable of pulling wider tool bars at greater speeds across fields, have often outrun capacities of control systems during late fall and early spring cold weather applications. This condition prevails not only with tool bar-mounted mechanical meters but also electrically-controlled meters due to unknown system resistance from the nurse tank to the tool bar mounted meter. See U.S. Pat. No. 4,364,409, Dec. 21, 1982, Fluid Flow Control Device, James S. Jones, the disclosure of which is incorporated herein by reference.
Thus, as ammonia is fed through the application system by its own vapor pressure, it experiences an ever changing set of conditions as it moves through the system. A system at rest (no product moving in or out) has a thermal balance; the thermal energy in equals the thermal energy out. On demand of product the product moves up the dip tube forming an efflux at the entrance of the dip tube. This acceleration requires a drop in temperature reflected by the dew point of the product at the tank pressure and the dew point of the product that has been accelerated a the entrance of the dip tube. This drop in temperature is provided by the formation of vapor within the system. This reaction is repeated at every restriction as the product moves through the system--the withdrawal valve, the hose end valve (or reader), the safety coupling, the shut-off valve, the meter (or throttle), the manifold outlet fittings and the discharge tube on the knives. Unlike a true liquid system where the points of restrictions can be considered orifices in series and where one value can be established for all of the points of restrictions, an ammonia system must deal with each restriction individually.
It is known to utilize vapor stripping prior to metering, which works very well in the mid-upper range of application. The gray area is in the low range of applications where stripping dumps first open. Over-stripping may occur, which reduces the system's dew point by refrigeration, and ammonia across the metering point behaves like a liquid rather than a liquified gas resulting in over-application at lower application rates on small grains using narrow tool bars. See U.S. Pat. No. 4,657,568 Apparatus For Volumetrically Controlling The Flow Of A Gas And Liquid Mixture, James S. Jones, the disclosure of which is incorporated herein by reference.
Prior apparatus for dividing flow has been acceptable for high outputs required for larger grains, grown usually in 30 inch rows and requiring 200 pounds N or more per acre; however, uneven distribution is not uncommon with small grains with larger tool bars on the rolling hills often found in wheat country. A 45 foot tool bar used for corn, with 30 inch rows having 18 knives applying 200 pounds N per acre, is a much simpler distribution problem than the same tool bar used for wheat, with 12 inch rows, 45 knives, and 75 pounds N per acre. A two manifold system has been used with corn with limited success; however, in wheat, two manifolds with the present systems becomes a disaster. The best present art is a controlled manifold. See U.S. Pat. No. 4,807,663, David Ward and James S. Jones, Feb. 28, 1989, the disclosure of which is incorporated herein by reference.
In an effort to even out the pressure difference across each row a constant hose length for each row is typically used. So, with a 45 foot tool bar in corn, in excess of 400 feet of applicator hose is required; with wheat, in excess of 1,000 feet is required.
A fairly common problem in applying ammonia is the finishing off of a field with a strip less wide than the width of the tool bar. For example, a farmer may apply an entire field in 40 foot wide strips with a 40 foot wide tool bar, but end up with a 30 foot wide remainder. Presently, his only choices would be to leave the remainder untreated or to overtreat a 10 foot strip.